The use of floating roof tanks for storage of crude oil and petroleum products (liquid hydrocarbons) of more than 1.5 PSIA true vapor pressure has become a preferred practice in the oil industry for a number of years. For the most part, this is due to a reduction of hydrocarbon vapor emissions and improved safety of floating roof tanks in general when compared with cone roof tanks.
The floating roof tank achieves a high percentage reduction of evaporation loss, as compared with an atmospheric pressure tank, inasmuch as the floating roof floats upon the product and the air space in contact with the volatile liquid is almost completely eliminated. Accordingly, the small air space left becomes easily saturated and prevents further substantial vaporization of the liquid hydrocarbon into the saturated air space.
Floating roofs are of three general types: pan, pontoon and double deck. Various versions of these basic types of roofs are manufactured and are tailored to emphasize some particular feature such as vapor trapping, full liquid contact, load carrying capacity, or roof stability. Selection of a specific type of roof depends upon the properties of the product stored, particularly vapor pressure and corrosive nature, and the roof stability required under service conditions. The least costly pan type roof is also the least stable and offers the least protection against product heating. The more costly double deck roof provides good stability as well as insulation to minimize the excessive heating of stored products. Heating of products should be avoided because it is the cause of excessive evaporation loss.
All of the classes of floating roofs mentioned provide an annular rim space between the tank shell and the floating roof to permit installation and maintenance of the sealing mechanism. A sealing mechanism is necessary because the floating roofs cannot be made to precisely fit within the confines of the storage tank itself.
In order to obtain the full benefit of a floating roof as a vapor conservation device, the annular rim space between the storage tank and the floating roof must be fitted with a tight seal. A good sealing element will close this space effectively while permitting normal roof movement due to loading and unloading as well as to diurnal expansion and contraction, the latter being due to an increase and decrease in liquid volume within the tank due to an increase in temperature during the day and a decrease in ambient temperature during the night. In this manner, the floating roof moves upwardly and downwardly slight amounts during the course of a normal 24 -hour period. Another reason for a seal between the floating roof and the tank structure itself is the lack of absolute perfection in the building of the tank resulting in an out-of-roundness which a dimensionally static seal would not be able to accommodate. Moreover, the mechanism that provides the sealing force for most seals also serves to keep the floating roof centered in the middle of the tank with substantially equalized pressure about the sealing member.
Two types of annular sealing members have been provided in the past. These have taken a form of both metallic as well as nonmetallic seals. Due to the general lack of use of metallic seals in the last few years, only nonmetallic seals wll be discussed. Nonmetallic seals are a relatively recent development in the search for more effective sealing assistance. They were used sparingly many years ago, but have come into prominence in the past 10 to 15 years. The identifying characteristic of nonmetallic seals is that they use a coated fabric band in sliding contact with the tank shell. Liquid pressure, gas pressure, or resilient foam are sometimes used to provide the necessary forces to expand the seal against the tank shell and provide a more efficient sealing system.
Nonmetallic seals have two major advantages: flexibility and elimination of the large undesirable annular air vapor space normally prevalent in steel shoe type sealing systems. The flexibility of nonmetallic seals results in a better conformity to the tank shell and, consequently, a better seal. Normally, the seals are placed in direct contact with the stored product which prevents the occurrence of any vapor space in the tank and, consequently, eliminates breathing loss. The seals themselves are generally manufactured from a fabric which may have any one of a number of different specifications. Due to the fact that all fabrics contribute to permeation losses, the area of fabric within any sealing arrangement should be minimized. Seal manufactures have recently developed as many fabric products for nonmetallic seals as there are needs.
Nonmetallic seals have, heretofore, been susceptible of one major criticism. Specifically, the flexible or nonmetallic seals do not exhibit as much potential contact area with the inner surface of the storage tank as do metallic seals. Any gap extending to the product surface may easily cause vapor less to the ambient atmosphere. Moreover, wind and wind eddying increases this loss.
The annular rim space between the tank shell and the floating roof is the principal source of evaporation loss. Heretofore, it was suggested with some certainty that there are only two major ways in which evaporation loss occurred in the seal area of a floating roof tank. These were evaporation through the space between the seal and the tank shell which might be referred to as a diffusional loss and a vapor permeation through the sealing fabric. It has also been shown that a third major area of concern regarding evaporation losses from floating roof tanks is that of evaporation of liquid left on the inner surface of the storage tank subsequent to movement of the floating roof downwardly due to diurnal contraction and/or liquid unloading. Due to the fact that presently available sealing systems do not anticipate this as a major source of evaporation loss, they are ineffective in their attempts to substantially reduce evaporation losses resultant from this cause.
The losses between the seal and the tank shell may occur by evaporation from exposed product surfaces or from product wicking up the tank shell or both. Product exposure may result from poor fitting seals, or may occur at shell irregularities, e.g. rivet heads or shell discontinuities. Vapos which form between the sealing ring and the tank shell must travel upward through the depth or vertical length of the seal to escape. For a given gap, the deeper the sealing element, the more effective the seal in preventing loss. As noted previously, wind increases this loss. Under the force of wind action, air enters the space between the seal and the tank shell, sweeping out vapor at a rate which increases with the wind velocity. Furthermore, the play of wind across the roof of the tank may cause eddy currents with resultant areas of reduced pressure near the rim of the roof. These areas of reduced pressure induce flow past the seal. Laboratory tests have shown in many cases the losses from vapor permeation through the sealing fabric are so small as to become negligible.
The quantitative amounts of losses due to evaporation from floating roof storage tanks have generally been tolerated in the past. However, recently governmental regulations have tended to become more restrictive in hydrocarbon vapor emissions to the point that heretofore "small" emissions from floating roof tanks are considered excessive. As a result, the rather ineffectual sealing systems of the past have become even more unacceptable due to the more stringent requirements being promulgated by governmental agencies. Consequently, a new sealing system, based in part upon a new evaporation loss theory, was devised.